1. Field of the Invention
This invention relates to measuring optical wavelengths. More particularly, the present invention relates to wavelength reference systems suitable for use in fiber Bragg grating (FBG) sensor systems.
2. Description of the Related Art
A fiber Bragg grating (FBG) is an optical element formed by photo-induced modulation of the refractive index of the core of an optical fiber. An FBG element reflects light having a wavelength within a narrow bandwidth that is centered on a wavelength that is referred to as the Bragg wavelength. The Bragg wavelength depends not only on the optical fiber itself, but also on physical parameters such as temperature and strain that impact on the refractive index of the optical fiber. Therefore, FBG elements can be used as sensors to measure such physical parameters since, after proper calibration, the Bragg wavelength provides an absolute measure of the physical parameters.
While FBG elements are highly useful sensors, in most applications the Bragg wavelength has to be measured with a resolution, repeatability and accuracy of about 1 pico-meter (pm). With a Bragg wavelength of 1.55 microns, a shift of 1 pm corresponds to a change in temperature of approximately 0.1° C. Because of the required accuracy of the Bragg wavelength determination, some type of reference wavelength measurement system is usually required.
As previously indicated, because the Bragg wavelengths must be very precisely determined, an accurate reference is usually required. Making the problem of determining Bragg wavelengths more difficult is the fact that broadband sources and tunable filters are subject to gradients and ripples in the filtered light source spectrum that can induce small wavelength shifts in the measured peak wavelengths. This leads to uncertainties in the measured Bragg wavelength.
In practice, the Bragg wavelengths of one or more FBG elements are often measured using a broadband light source, for example an edge-light-emitting diode (ELED) or a superfluorescent fiber source (SFS), in combination with a tunable optical filter, for example a piezoelectric transducer (PZT) tunable fiber Fabry-Perot filter [see, for example, Kersey, A. D., Berkoff, T. A., and Morey, W. W., “Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filter”, Optics Letters, Vol. 18, pp. 1370-1372, 1993]. Alternatively, Bragg wavelengths can be determined using a tunable laser source, for example, an external cavity semiconductor laser with a tunable FBG reflector. Such as described in U.S. Pat. No. 5,401,956, issued on Mar. 28, 1995. Either way, an applied light source is swept over a range of wavelengths and the reflected wavelengths, which correspond to the Bragg wavelengths of the elements, are determined.
FBG sensor systems usually include a wavelength reference system to assist determining the Bragg wavelengths. Such reference systems are often based on a fixed cavity length interference filter, typically a fixed Fabry-Perot wavelength filter, and at least one reference FBG. When the wavelength swept light is input to the fixed cavity length interference filter the output of the filter is a pulse train that represents the fringes/peaks of the optical transmission, or of the reflection spectrum, of the filter, i.e., a comb spectrum having constant frequency spacing. This wavelength reference system reduces problems associated with non-linearity, drift and hysterisis. The reference FBG element can be used either for identification of one of the individual interference filter comb peaks, which is then used as the wavelength reference, or for relative wavelength measurements between FBG sensor elements and the reference FBG. Thus, the comb spectrum establishes a frequency/wavelength scale.
The resonance wavelength of a fixed Fabry-Perot wavelength filter can shift by about 13 pm/° C., which is about the same as the wavelength shift of an unstrained FBG. By calibrating both the comb peak wavelengths of the reference fixed Fabry-Perot wavelength filter and of the reference FBG vs. temperature, and by accurately measuring the temperatures of the fixed Fabry-Perot wavelength filter and of the reference FBG, the Bragg wavelengths of the FBGs sensors can be accurately determined. Alternatively, the temperatures of the fixed Fabry-Perot wavelength filter and of the reference FBG can be stabilized using an oven or an ice bath.
One problem that occurs when using one or more reference FBG elements is that they occupy one or more wavelength bands, which limits the total available bandwidth, and hence the number of wavelength multiplexed FBG sensor elements that can be used. Additionally, reference FBG elements add complexity and cost to the overall system.
Therefore, a device and method for accurately measuring optical wavelengths, in particular reflected Bragg wavelengths of one or more wavelength multiplexed FBG elements would be beneficial. An accurate wavelength reference system that enables accurate wavelength measurements over a wide wavelength range, and which is relatively insensitive to changes in temperature over a wide range of operating temperatures, would also be beneficial. A new FBG sensor system that does not require the use a reference FBG would be particularly useful.